Displacements of a shaft axis occur at frequencies comparable to or greater than the frequency of rotation of the shaft in many machines and apparatus. A shaft which supports a grinding wheel in a grinding machine is subject to such displacement both as a result of imbalance of the rotating masses and as a result of external forces applied to the grinding wheel when it engages a workpiece.
One known method of measuring displacement uses a probe to mechanically follow he shaft and a transducer to measure the displacement of the probe. For complete information two probes at right angles are required.
The present invention uses a known method of measuring angular displacement of a shaft. The method is applied at two or more angularly spaced locations and the resulting measurements are processed to produce a measurement of relative displacement of the shaft in the required transverse direction. Two locations are needed to measure displacement in a given direction. Three or more locations are necessary to provide full information about the displacement of the shaft.